The air in the cabin isn’t sealed in. Fresh air is continuously introduced during the flight. A plane’s jets are already sucking in and compressing huge volumes of air to burn with the aviation fuel. Some of this is diverted for the passengers to breathe. Because the compression heats up the air, it must first be ducted around the wings to be cooled down. The air already in the cabin is passed through high-efficiency particulate (HEPA) filters to remove bacteria and viruses and then mixed 50:50 with the fresh air from outside. The excess cabin air is vented through valves to the rear of the plane to keep the cabin pressure constant.

This system means that the air in the plane’s cabin is completely replaced around 15 times an hour, but this is mainly about controlling the temperature and removing contaminants. The oxygen that all the passengers breathe is less than one per cent of the fresh oxygen entering the cabin.